RINGWORLDS

Ringworlds are enormous artificial worlds constructed in a circular shape thousands or millions of miles in diameter. They share some similarities to Planetary Haloes, but either are independent space structures or encircle stars instead of planets.

The idea was first introduced and popularized in Larry Niven's seminal 1970s science fiction novel Ringworld. His artificial world that circled a sun remains the prototypical definition of a megastructure, and is perhaps the second most famous example of such a structure after the Death Star from the Star Wars film series.

This structure comes from the Halo series of computer games by Bungie Studios. It is a circular ribbon of super-strong material some 10,000 miles in diameter and 320 miles wide in orbit about a gas giant planet. The structure is set spinning to produce artificial gravity along its inner surface through centripetal acceleration. The inner surface is also sculpted to resemble a life-bearing world, complete with a breathable atmosphere, waterways, seas, land masses, forests, and so on, and stocked with lifeforms from various worlds. The total habitable surface area of a Forerunner Halo is about 10 million square kilometers, or slightly more than the surface area of Canada. Its average thickness is a little over 22 kilometers, much of the volume not dedicated to the habitat given over to maintenance machinery and other mysterious technology.

In the Halo universe, seven of these constructs are known to exist, built many thousands of years ago by a mysterious alien race called the Forerunners to contain and study a parasitic alien lifeform called the Flood.

Structures such as the Halo are under constant enormous structural stress. Remember that the Halo is basically the size and mass of a continent, spinning so that each section wants to rip away from the others and go careening into space just from pure centrifugal force. No normal kind of rock, crystal, or metal could stand up to those kind of stresses, no matter how thickly made. And unlike a planet, it does not have a planet's gravitational field to counteract this force. So in order to prevent the ring from flying apart, the outer shell of the Halo is constructed from some unknown Ultra-Tensile-Strength material, on the order of thousands of times the resiliency of carbon steel, just to keep the artifact together. The even larger habitable artifacts in this class, Orbitals and the Ringworld, require materials of even vastly greater strength to hold themselves together.

Ringworlds of this size may arise from Planetary Halo technology (not to be confused with name of the computer game here; see separate article on Planetary Haloes.) Planetary Haloes are rings constructed around terrestrial planets, often arising as a consequence of Space Elevator technology. Numerous space elevators are built around the rotational equator of planet, and the ring is slowly built up in geosynchronous orbit where each elevator system's center of gravity is located.

In this case, the ring is specially reinforced and designed to be mobile, instead of just acting as a static in-orbit habitat. When complete, the ring is detached from the mooring space elevators and very slowly nudged away from the planet along its poles. Once clear, the new independent Halo can be spun up for gravity and maneuvered into a new orbit.

This scheme would allow a steady stream of building material to the new Halo for minimal cost. Unlike other means of building a ringworld, or most megastructures, if built around a life-bearing world it does provide a means by which megatons of organic material--topsoil, breathable atmosphere, plant life, and so on--can easily be transported up to fill the habitat in a short period of time.

The Forerunner Halo may or may not have used this technique; if so, it would have to have been constructed around a much smaller, slower-spinning world than Earth, such as a moon. Since Forerunner Haloes have so far been seen only in orbit about gas giants, this would seem to follow. A Halo built around Earth with this method would have a diameter of about 52,000 miles, about five times the size of the Forerunner artifact.

The Tech Level of the Forerunner Halo as a habitat is 18, but there are number of indications that the Forerunner civilization was actually more advanced than this, and that the Haloes may in fact hold a number of ultra-tech secrets not yet revealed.

GOSSAMER RING
Tech Level: 19

Perhaps the most practical type of ringworld to construct is a so-called gossamer ring--a structure encircling a star made up almost entirely of a sparse framework and micron-thin solar cell material. It sole purpose would be to gather energy from its parent star and beam it via microwaves or laser to other worlds or structures nearby.

Unlike other artifacts of this type, the gossamer ring does not need fantastically exotic materials for its construction. A thin frame work of material made of carbon nanotube filaments would probably prove workable, as would the ultra-thin but strong solar sails material stretched between the frame work would suffice in converting sunlight into power.

Because gossamer rings would be constructed solely for the purpose of energy gathering, they can be constructed much closer to the sun than their larger, life-supporting cousins. The ring could be from several thousand to a million or so miles wide, depending on how much sunlight it is designed to capture, and would most likely have a relatively tight radius around its parent star, probably some 25 to 40 million miles, in order to maximize its energy absorption per square meter. Much farther in, the ring would risk deformation and damage from the likes of intense solar winds, radiation, and solar storms. This of course assumes a sun-sized star. Different stars with different diameters and radiative output would have different minimal radii for gossamer rings.

Both light pressure from the parent star as well as its own rotational momentum would be enough to keep the structure stable and rigid. The gossamer ring has much in common conceptually with Statites, and may indeed be a natural long-term outgrowth of that technology. Solar-orbit statites may steadily grow in size and number until linking them together in a chain around the sun would seem the next logical step.

Like most rotating structures in space, it would be subject to progressional instability, where it would slowly slide off center in the plane of its orbit if left to its own devices too long. The presence of attitude thrusters would help offset this. However, the ring could also heighten or lower its reflectivity over large sections of its sunward-facing surfaces, using light pressure to very slowly correct slight perturbations in its orientation around its parent star.

Gossamer rings would also require much less building material than the larger ringworlds; maybe only the mass of a small moon would be required for construction. The shadow squares of Larry Niven's Ringworld novels have some aspects of a gossamer ring.

Orbitals are rings of ultra-tensile-strength material some fourteen million kilometers in diameter and between one thousand and six thousand kilometers wide. They are spun up to simulate gravity along their inner surfaces, which are lined with soil and water and a breathable atmosphere in order to support life. The edges of the inner surface are lined with walls hundreds of kilometers high in order to keep in the atmosphere.

Orbitals circle their parent star much like a normal planet, but are canted up at an angle to it, so the back half of its inner surface receives sunlight while the other half remains in shadow. Since the Orbitals are spun up to complete a cycle in approximately 24 hours, this creates a very convenient day-night cycle for its inhabitants.

Orbital rings are separated into individual "plates", which can be thought of as individual continents. Different plates can have wildly varying characteristics from one another. Some plates may closely resemble the natural habitats found on normal life-bearing worlds. Others may have unusual structures or features of a more wild or artistic bent, such as kilometer-high mountain-bridges over large expanses of water or floating colonies on an Orbital-spanning river.

Orbital spin rates need to be high, not only to provide a convenient day-night cycle and the right amount of gravity but to keep the entire structure stable and rigid. Because of the immense stresses this causes on such a huge mass, ultra-tensile-strength materials are needed for its construction, which in the Culture universe are also supplemented by various force fields and quantum force manipulation. This super-material is thought not to be quite at the same level of toughness as scrith (see below), but it would still be thousands of times more resilient than any material currently known. Even so, if Orbitals were ever to lose power on a large scale that its structural support force fields would collapse, there's a good possibility that the structure would deform and possibly even tear itself apart just from centrifugal force.

The constructs are each overseen by a highly advanced and powerful AI called a Hub Mind, which tends to both active maintenance and overseeing the daily needs of the Orbital's inhabitants.

RINGWORLD
Tech Level: 23

The view from the surface of the Ringworld. Art (c) Michael R. Madsen.

The Ringworld, the most famous locality in Larry Niven's Known Space future history, is built on a truly colossal and mind-staggering scale, even more so than the other artifacts mentioned here. A million miles wide, it wraps around its parent star at a radius about equal to Earth's distance from the sun, giving it a total circumference exceeding 530 million miles. It would take one hundred Forerunner Haloes side by side just to equal the width of the Ringworld; fifty thousand plus to line up along its length. The Ringworld would require the disassembly of an entire solar system, as well as fantastically advanced technologies such as large-scale matter conversion, to construct.

Like an Orbital, the Ringworld's inner surface is sculpted to resemble that of natural life-bearing worlds. Walls a thousand miles high line the ring's inner surface in order to keep in the atmosphere. It has a habitable surface area approximately equal to three million Earth-sized planets.

In order to simulate close to earth-normal gravity along its inner surface, the ring is spun up to 770 miles per second, over 4000 times the speed of sound. Just getting the Ringworld up to this level of rotation would be in itself a colossal feat; it would require the conversion to pure energy of about twenty Jupiter-sized planetary masses, or several hundred years of the entire energy output of the parent star.

Such an enormous mass spinning so quickly creates enormous structural stresses, and the builders in turn would need materials of insane tensile strength to hold the ring together. Larry Niven provided such a material in scrith, an ultra-tensile-strength material that is 50,000 times more resilient than the toughest steel alloy. The material seems to somehow tap into the strong nuclear force that binds the particles in atomic nuclei together, and extends it onto a macroscopic scale.

The floor of the Ringworld is made up of some 30 meters or so of solid, sculpted scrith, with another kilometer below it composed of a highly foamed aerogel-like scrith variant meant as meteor defense. The Ringworld has a mass similar to Jupiter, the vast majority of which is scrith.

Scrith has a number of unusual properties beyond its super-resiliency. It blocks 40% of all neutrinos passing through it, particles that would otherwise ghost through a hundred light years of lead effortlessly. Its surface is nearly frictionless. It absorbs most electromagnetic radiation that strikes it (usually appearing matte black to an observer) and can even block carrier waves in hyperspace. How exactly it is able to do all this is never really explained, but there is obviously supremely sophisticated quantum force manipulations going on. Unlike the materials needed to build the Culture Orbitals, however, scrith appears not to need supplementary force fields nor a constant stream of energy to maintain its structural integrity.

Scrith may be sculpted and permeated by mysterious devices called cziltang brones. It allowed inhabitants of the ring to move back and forth through the pure scrith of the rim walls to outer structures such as spaceport ledges.

Like many spinning megastructures, the Ringworld suffers from a vulnerability to progressional instability. Huge arrays of bussard-ramjet-like attitude jets were mounted on both rims to counteract this. However, one of the native civilizations that arose on the ring dismounted many of these to use as drives on starships, and as a result the Ringworld came precariously close to sliding into its own sun. Dealing with this crisis formed one of the major plot points of the second novel, The Ringworld Engineers.

A 30-hour day/night cycle is provided on the surface of the Ringworld by a series of twenty shadow squares which orbit in much closer to the sun. They measure 1.6 million kilometers by 4 million kilometers, and are spaced 9.6 million kilometers apart. They are held together by long lengths of ultra-tensile wire similar in many ways to carbon nanotube filaments. The shadows cast by the squares can be seen very slowly sliding over the surface of the ring, providing ten hours of night at a time to surface locations.

Over half the Ringworld's habitable surface is water. Two great Oceans exist on either side of the Ring, each stretching from one rim wall to the next, making them nearly a million miles wide. Thatís large enough to swallow the planet Saturn if the oceans were made deep enough. Among the many near-uncountable number of islands in these oceans are "maps" of known homeworlds of different sentient species located in the ring's general galactic neighborhood. These Maps recreated the continents of these worlds on a one-to-one scale and stocked them with lifeforms from them. Among these were Maps of Earth and Mars. All the recreated continents of Earth grouped together looking like nothing more than a tiny archipelago in this vast watery basin.

The Map of Mars reached over 40 miles into the atmosphere on an enormous pillar of scrith in order simulate normal Martian air pressure. The Map of Mars is also hollow, serving as an enormous Control and Repair center for the entire ring. If other such control centers exist on the ring is unknown.

The landscape of the ring is constantly eroding much like that on natural worlds. It is continually renewed by a series of colossal recycling systems. Silt and mud are collected at the bottom of oceans and seas and are transported to the tops of some 46,000 'spill mountains' equally spaced along both rim walls. Each spill mountain is between 50 and 60 kilometers high and appear to be regular half-cones of rocky material leaning against the rim wall.

The builders of the Ringworld worried quite a bit about possible meteoroid impacts, and apparently with good reason. In the novels, the explorers encountered two enormous punctures in the floor of the ring caused by massive impacts. One had hit the inner habitable surface dead-on, creating a small rupture through which atmosphere was venting out into space (though it would take millions of years for it to drain enough air to present a real hazard to the ring as a whole.) Around this first puncture formed a hurricane-sized eye-shaped storm. The second puncture hit the ring from underneath, a moon-sized mass falling into the system from interstellar space. It created a vast deformation in the scrith that reached up even past the rim walls before it broke through. The natives later called this thousand-mile high deformity Fist-Of-God mountain.

In order to protect against such disasters, the builders created the Meteor Defense System. The floor of the Ringworld is interwoven with a gigantic grid of powerful superconductors. When fully charged, this grid manipulates the parent star's magnetic field, pulling off from it a gigantic solar prominence and squeezing and manipulating it into the universe's most gigantic gas laser. A typical shot by the Meteor Defense Laser (MDL) delivers 3 x 10^27 ergs, enough to vaporize away the outer crust of a planet and destroy just about any artificial structure or material short of scrith. Worse, it can be fired continuously until the approaching threat is completely vaporized. The beam is most powerful when aimed at the floor of the ring, but by sacrificing some of this potency the superconductor grid can manipulate the beam to fire in just about any direction, much to the horror of a number of would-be explorers from Known Space in the later novels. The grid can even manipulate its field enough that the beam can be diffracted somewhat over the rim walls to hit at targets approaching the underside at great distances, but at much diminished power. The only "safe spot" within the Ringworld system from the MDL is directly under the Ring floor.

The MDL was left on automatic for many thousands of years, programmed to fire at any object moving across the surface of the ring at speeds greater than 7.04 kilometers per second. A great many civilizations that arose on the ring were annihilated by the automated MDL when they unknowingly developed technologies that could reach that speed.

The landscape as seen form the surface of the ring would be quite an incredible sight. There would be no distinct horizon; land would just go on and on in all directions until it fuzzed in the distance with the distorted haze induced by hundreds of miles of atmosphere. On either side would rise the Arch, the distant reaches of the ring looking like an enormous cosmic hoop rising from the distant horizon-haze, with the sun hanging in its center. Twilight and dawn would take on the aspects of solar eclipses, as the shadow square would slowly slide across the disc of the sun.

The Ringworld was built by the Pak, a species that was as highly intelligent as it was highly xenophobic and fanatical in its pursuit to protect its genetic bloodline at all costs. In Niven's universe, they also colonized Earth and were the ancestors of modern homo sapiens. The Pak were supremely warlike, and there are indications that the Ringworld was built so large in order to give each Pak bloodline near-infinite room to grow and prosper without infringing on the others, and thus ending the constant wars that had always threatened their species with annihilation.

The Pak also originally evolved near the galactic core, a region in the Known Space universe that frequently erupts in a galaxy-wide explosion caused by chain-reactions of tightly-packed supernovae. The Ringworld with its near-indestructible scrith armor may also function as a fortress against these explosions, allowing for the very long-term survivability of the Pak species.

However, the Pak Protectors, the super-genius, superhuman form of the race, died out in a plague, leaving their breeders (who much resembled humans' prehistoric ancestors) free to run wild over the surface of the Ringworld habitat. Over hundreds of thousands of years, they evolved to fill a number of different ecological niches. Thus, the ring is inhabited by a great many of humanity's long-lost hominid cousins. The Ringworld at the time of the novels was estimated to hold some 30 trillion sentient inhabitants.

Though smaller than other types of megastructures, such as certain types of Dyson Spheres, the presence of ultra-tech features such as scrith, the MDL, and its colossal spin rate nudges the Ringworld to a slightly higher tech level.